This invention relates to the attenuation of pressure pulses in a liquid. More particularly, it relates to a method and apparatus for attenuating the pressure pulses in a liquid travelling in a pipe, such as an aqueous stock slurry travelling from a pump through a pipe into a papermaking machine headbox. Still more particularly, this invention relates to an active type of attenuator wherein the pressure pulses are attenuated by positive action by the attenuator as opposed to so-called passive attenuators wherein pressure pulses are attenuated by reflective movement of the attenuator apparatus responsive to pressure pulses in the travelling liquid.
Examples of so-called passive attenuators are shown and described in U.S. Pat. Nos. 4,030,971 and 4,262,700. Such passive attenuators operate by permitting the pressure pulse to distend a flexible diaphragm which is biased by pressurized gas on its opposing side. The passing pressure pulse thus dissipates its pressure against the biased diaphragm and the pulse is diminished or eliminated downstream of the attenuator. These passive types of attenuators operate well under certain operating conditions, but they also have some inherent limitations. For example, they are slow to respond to rapid fluid pressure pulsations and cannot attenuate pulsations over a broad range of frequencies, such as about 0.5 Hz to about 100 Hz. Passive attenuators also have a propensity to short circuit since vibration is transmitted structurally through the pipe as well as through the liquid travelling through the pipe. Thus, the pressure pulsation can pass from upstream of the attenuator to downstream of the attenuator through the attenuator structure even though part of the pulsation travelling in the liquid may be attenuated. Finally, passive attenuators tend to saturate at changing frequencies and pressure pulse amplitudes rather than adapt to changing conditions. To saturate is meant to experience increased fluid pressure to a point where the attenuator diaphragm is moved against its stops and maintained there so as to render it ineffective in responding to fluid pulsations.
This invention actively attenuates pressure pulses in a confined travelling liquid, such as pulp stock being pumped through a pipe into a papermaking machine headbox. Pressure sensors are attached to the pipe to sense pressure pulsations within the pipe at one or more selected locations between the fan pump and an actuator attached to the stock pipe immediately before it enters the papermaking machine headbox. These pressure input sensors are electrically-linked with a pressure transducer amplifier system connected with a controller control monitor which measures the responses and creates a model of the pressures and frequencies according to these measurements. The controller then signals a power amplifier which is connected to the actuator in the pipe immediately adjacent the inlet into the headbox. The actuator utilizes a flexible diaphragm, such as a rubber membrane mounted flush with the pipe interior wall, to produce pressure pulses which are equal in frequency and amplitude to the pressure pulses desired to be nulled, but phase reversed so as to interfere with the pressure pulse in the liquid in the pipe and null the effect of its pressure.
The input pressure sensors can be located both upstream and downstream of the actuator and separate error sensors located in the headbox and recirculation pipe outlet at the other side of the headbox can be provided to monitor the effect of the attenuating operation. The pressure pulsations sensed by the controller can therefore have travelled upstream and downstream in the pipe.
This active attenuator is a power attenuator consisting of power amplifier and actuator in that it produces pressure pulses based on the pressure pulses measured travelling in the liquid in the pipe. This attenuator can track and produce nulling pressure pulsations over a wide spectrum range of typically about 0.5 Hz to 100 Hz and achieve attenuation of typically about -10 to -30 dB of both random and periodic frequencies within the spectrum range measured. The controller, or control monitor, models all pressure pulsations received by the input sensors into a composite synthesized waveform which, in turn, is utilized by the power amplifier to drive the actuator to operate a flexible diaphragm which reproduces the pressure pulses in phased reversed form into the liquid in the pipe. The continuous monitoring and control system permits the attenuator to respond to both the periodic pressure pulsations created by rotating machinery, such as the fan pump, as well as intermittent, or random, pulsations produced by system resonances, turbulence, valves, etc.
Attenuation of pressure pulses in the stock inlet pipe carrying pulp stock into the headbox of a papermaking machine is of great importance. Upstream of the headbox are many machine components which produce, refine, screen, distribute and pump the aqueous pulp stock into the headbox. The operation of each of these machine components produces some sort of disturbance in the stock which is manifested by a random or periodic pressure pulse in the stock. The practical effect of this pressure pulse is that when the stock is projected from the headbox slice opening onto the forming wire, there are small variations (or fluctuations) in the rate of stock ejection and these produce corresponding variations in the basis weight, formation, and thickness of the paper on the forming wire as the water is drained from the stock. This phenomenon is know as "barring" and can be readily perceived in the finished paper product. Since barring deleteriously affects paper quality, any reduction is highly desirable.
Accordingly, it is an object of this invention to provide a method and apparatus to actively attenuate pressure pulsations in a liquid travelling in a confined space to about -10 to about -30 dB in a spectrum range of about 0.5 to about 100 Hz.
It is another object of this invention to provide a method and apparatus to actively attenuate both random and periodic pressure pulsations in a liquid travelling in a confined space.
Yet another object of this invention is to provide a method and apparatus for actively reducing amplitude fluctuations by a factor of 2 of input fluctuations, or more.
Still another object of this invention is to provide a transducer amplifier system which receives signals from input and error sensors and utilizes them in conjunction with a controller to signal an actuator to produce nulling pulsations to attenuate pulsations in the liquid.
An advantage of this invention is its ability to quickly adjust to changes in operating conditions and provide the desired nulling pulsations.
These and other objects, features and advantages of the invention will become obvious to those skilled in the art when the specification and claims are read in conjunction with the attached figures.